Greek philosopher, pupil of Plato, tutor of Alexander the Great, and founder of the Peripatetic school at Athens; author of works on logic, ethics, politics, poetics, rhetoric, biology, zoology and metaphysics. His works influenced Muslim philosophy and science and medieval scholastic philosophy.

The Heretic's Feast: A History of Vegetarianism by Colin Spencer
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Extract from 'The Heretics Feast' by Colin Spencer:

The Platonic ambivalence is shared to an even greater degree by Aristotle. As he played a part in medieval Christian thinking, such ambivalence is rather to be regretted, for he was reverend and has helped shape many of our attitudes in society today. Aristotle denies the power of thought to animals, maintaining that they are capable only of sensation and appetite, and that they need the rule of humankind in order to survive. (Why or how animals flourish in the wild he appears not have considered.) In the Aristotelian view plants and animals exist for the use of humans. In one passage he equates animals with slaves, by saying the ways we use tame animals and slaves are not very different. (This, of course, merely illuminates how the Greek saw their slaves.) How very different from Pythagoras, who saw the immortal soul in everything, although even Aristotle admits: ‘we should approach the inquiry about each animal without aversion, knowing that in all of them there is something natural and beautiful.’

Aristotle also writes sympathetically of the Orphic view: ‘the poems known as Orphic say that the soul is born by the winds, enters from the air into animals when they breathe.’ This view of breath as akin to soul is close to the Hindu view and that of Homer, and Aristotle seems here to agree with Pythagoras that animals have souls. To be inconsistent was not a crime then, as it seems to be today - we must understand that the complexity of the world and the enthusiastic exploration of it were mirrored in such ambivalence.

Extract from a review of Animal Minds and Human Morals - The Origins of the Western Debate by Richard Sorabji. Review by Stephen Salkever:

Richard Sorabji's previous books in Greek philosophy include several important studies of Aristotelian and post-Aristotelian philosophy of science and metaphysics.

. . . Aristotle famously claims in the Politics that humans are the only animal capable of logos (though he elsewhere attributes forms of reason and intelligence to animals, for example in the first chapter of the Metaphysics), but his psychological theory as a whole is marked by a narrowing of the scope of reason relative to Plato, and a corresponding increase in the cognitive content of the power of perception (aisthesis), a capacity possessed by all animals. For Aristotle, many animals possess true memory (though not the human power to remind ourselves deliberately of previous perceptions), can learn, and perceive not only immediate sensations but connections, relationships, and rudimentary universals and action-orienting propositions: "Whether or not Aristotle's lion perceives the ox as an ox, it certainly perceives it as a meal" (p. 62). In addition to the power of intentional perception, Aristotle's animals are capable of both passion and voluntary motion, and so are not simply driven about by impulses beyond their control.

On Sorabji's account, the idea so familiar to us today that animals are little more than inanimate machines, responding mindlessly by innate impulse to environmental cues enters philosophy with the Stoics [founded by Zeno: 336-264 BC]. Their simplified picture of animal life serves to increase the distance between us and other animals, and to make laughable claims that we owe animals justice or any moral consideration at all. This Stoic position, taken over into Christian theorizing by Augustine, has dominated our thinking ever since.